It is a common practice to provide the brake systems of commercial vehicles with automatic adjustment to compensate for brake lining wear. In cam actuated brake systems, both drum and disk types, an actuating shaft such as a camshaft, is repositioned relative to the brake actuating arm to position the brake shoe lining relative to the brake disk or drum surface. This establishes a desired clearance and the length of stroke required to move the brake shoes from the released position to the engaged position in which the brake lining is firmly seated against the disk or drum. Slack adjusters are known which automatically sense the wear of the brake lining and automatically adjust the camshaft to compensate for the wear in response to brake operation.
A slack adjuster of the aforementioned type is described in the Cumming et al U.S. Pat. No. 3,949,840. This patent discloses a slack adjuster which comprises an actuating arm having a worm gear rotatably mounted therein and a link pivotally connected thereto for connection with a brake applying push rod for pivoting the arm. The worm gear is mounted on the camshaft for rotation thereof to apply the brake when the actuating arm is pivoted. A shaft is rotatably mounted in the arm and has a worm thereon in driving engagement with the worm gear. A slack adjusting mechanism is connected between the link and the shaft and comprises a plunger assembly having an axially movable rotary driving connection with the shaft. A detent is resiliently biased into engagement with a helical thread with buttress teeth on the plunger. The plunger is moved outwardly during brake application and ratchets a number of teeth over the detent as a measure of the adjustment required. Upon release of the brakes, the plunger is moved inwardly and the detent causes rotation of the plunger and hence the worm and worm gear to adjust the camshaft.
The Oliver U.S. Pat. No. 3,507,369 discloses an automatic slack adjuster which employs a torque limiting, one-way drive mechanism to angularly adjust a camshaft relative to an actuating lever during brake application movement of the lever. The Lodjic et al U.S. Pat. No. 3,526,303 also describes a slack adjuster using a one-way clutch and which is operative upon brake application to adjust the camshaft.
The Sander et al U.S. Pat. No. Re. 26,965 describes a slack adjuster with a one-way clutch which is operative upon brake release movement of the actuating lever to effect adjustment of the camshaft. This adjuster employs an external cam and requires space for movement in a direction normal to the movement of the actuating rod.
Other automatic slack adjusters are disclosed in the following U.S. Pat. Nos.: to Bostwick, 3,121,478; to Bostwick, 3,307,661; to Bostwick, 3,618,715; to Svensson, 3,351,164; to Leber, 3,371,755; and to Schultz, 3,990,546.
The prior art slack adjusters are known to have one or more disadvantages. Certain adjusters attempt to sense the clearance between the brake lining and brake drum during brake application but these depend upon friction clutches and do not operate uniformly from cycle to cycle. Other adjusters attempt to sense the stroke length required for full brake application but accurate adjustment is not achieved because of backlash due to dimensional tolerances and because adjustment is made in finite increments. Certain slack adjusters are difficult to install due to their size and the space required. Other automatic adjusters have external operating parts such as cams and ratchets which may be impaired in operation by road dirt and other elements. Also, some of the prior art slack adjusters fail to facilitate manual adjustment of the camshaft upon initial installation in the vehicle and subsequent periodic maintenance.
A general object of this invention is to provide an improved automatic slack adjuster for vehicle brakes which overcomes certain disadvantages of the prior art.